Snow is a popular topic in the Great White North. Julie wrote A Salute To Snowy Streets, while Reuben discovers What snow teaches us about roads .

I had a media inquiry a year ago on “Why we become such bad drivers when it snows”, I didn’t take it, but the question is interesting in a sense. Unlike the rain in southern California, it always snows in Central Minnesota, so this is a recurring question.

Several things happen when it snows:

1. Roads are slipperier and require longer braking distances. People recognize that roads are slipperier and give increased spacing (following headway in the jargon) to the car in front. Instead of following at a 2 second headway (remember the 2 second rule from Driver’s Ed), they may follow at a 3 second headway. Since there are 3600 seconds in an hour, a 2 second headway implies 1800 vehicles per hour (traffic engineers will note of course that capacities per lane on freeways are often greater than this in good conditions, implying a shorter than 2 second headway). A three second headway implies a service flow or capacity (Qmax) of 1200 vehicles per hour. If the underlying demand (those who want to use the bottleneck at that time) remains unchanged at 1800 vph (say it snowed surprisingly in the middle of the day), then instead of serving 1800 cars, a bottleneck would serve only 1200 in an hour. This implies a queue 600 cars long. That is non-trivial.

2. Roads are slipperier. People recognize that roads are slipperier and drive slower to reduce braking distances, especially on roads which curve.

Kyte et al. “The effects of poor weather conditions on free-flow speed on a rural Interstate freeway are considered. It was found that free-flow speed is affected by pavement conditions, visibility, and wind speeds. It is also suggested that poor weather conditions occur with some degree of frequency in a number of U.S. cities and that the effects of poor weather should be considered in such cases as part of capacity and level-of-service analyses.”

3. Roads are slipperier. People insufficiently recognize that roads are slipperier and instead of giving increased spacing choose to crash into the vehicle in front of them. This temporarily reduces capacity to zero as the drivers sort out the situation.

Khattak and Knaap “significant increase was observed when winter snow event injury and noninjury crash rates (crashes per million vehicle kilometers) were compared with equivalent winter nonsnow event injury and noninjury crash rates. The data were then analyzed for injury occurrence. Results of a logit model indicated that crash injury occurrence on Interstate highways in Iowa depended on traffic, road geometry, and number of vehicles involved in a crash. Another finding from the logit model was that crashes during snow events were less injurious compared with equivalent nonsnow event crashes. Snow event–specific crash data were then analyzed to study the effects of snow event elements (e.g., snowfall intensity) on injury occurrence in vehicular crashes.”

4. Snow does in fact reduce demand. People choose not to go out when it snows. Arthur Huang and I conducted some research on Minnesota travel patterns statewide and found these elasticities (so if it snows, there is a 5.9% reduction in demand and 63.9% increase in crashes in the 3am to 9am time period). The reduction in demand seems to be less than the reduction in capacity, so queueing increases on roads at or near capacity in the absence of snow.

A. Huang, D. Levinson / Journal of Safety Research 41 (2010) 513–520

Others have found significant results as well:

Datla and Sharma “The commuter roads experience lowest reductions in traffic volume due to cold (up to 14%) while the recreational roads experience highest reduction (up to 31%). Impact of cold on off-peak hours (-10% to -15%) is generally higher than peak hours (-6% to -10%) for commuter roads and an opposite pattern is observed for recreational roads (peak hour reductions of 30–58% and off-peak hour reductions of 18–30%). A clear indication of reduction in traffic volume due to snow is also observed for all types of highways.”

So I wouldn’t say we become bad drivers. We are bad drivers, we just reveal it when the environment changes to the unexpected.

(This presents one more argument for robot cars. They can’t overcome the physics of braking distance or eliminate congestion, but they can in principle better assess road conditions and be less likely to crash.)

References:

Al Hassan, Y., and Derek J. Barker. “The impact of unseasonable or extreme weather on traffic activity within Lothian region, Scotland.” Journal of Transport Geography 7.3 (1999): 209-213.

Huang, Arthur, and David Levinson. “The effects of daylight saving time on vehicle crashes in Minnesota.” Journal of Safety Research 41.6 (2010): 513-520.

Khattak, Aemal J., and Keith K. Knapp. “Interstate highway crash injuries during winter snow and nonsnow events.” Transportation Research Record: Journal of the Transportation Research Board 1746.-1 (2001): 30-36.

Datla, Sandeep, and Satish Sharma. “Impact of cold and snow on temporal and spatial variations of highway traffic volumes.” Journal of Transport Geography 16.5 (2008): 358-372.

Kyte, Michael, et al. “Effect of weather on free-flow speed.” Transportation Research Record: Journal of the Transportation Research Board 1776.-1 (2001): 60-68.

Rooney Jr, John F. “The urban snow hazard in the United States: An appraisal of disruption.” Geographical Review (1967): 538-559.

Smith, Brian L., et al. “An investigation into the impact of rainfall on freeway traffic flow.” 83rd annual meeting of the Transportation Research Board, Washington DC. 2004.
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